At present, gas-hazardous mines extensively use systems for remote measuring of the methane concentration in the mine atmosphere and automatic gas protection systems which, in turn, employ thermocatalytic sensors. A methane sensor is the most critical unit of such systems, therefore, periodic diagnostic checks of such sensors are necessary for effective operation of such systems. In the course of a diagnostic check of a sensor one is expected to find out if the sensor operates normally under all operating conditions; one also finds out the causes of failures and pinpoints inoperative parts and units.
Modern practice offers no diagnostic checking methods that could fully solve the problem of checking thermocatalytic methane sensors employed in remote measuring and automatic gas protection systems in mines.
There are methods which provide a partial solution to the problem by making it possible to check the correctness of readings of thermocatalytic methane sensors. This is done by comparing readings of the sensor under steady-state gas conditions with those of a gas analyzer regarded as a standard instrument, with the gas analyzer and the sensor operating in the same medium (cf. "Apparatura systemy avtomaticheskoy gazovoy zashchity i tsentralizovannogo teleavtomaticheskogo controlya methana AMT-" /"Equipment Incorporated In the Automatic Gas Protection and Centralized Remote Methane Content Measuring System of the AMT-3 Type"/, Service Manual prepared by the Krasny Metallist Electromechanical Works in Konotop, Sumy Regional Press Department, Konotop, 1971, p. 32). The known methods do not make it possible to check the performance of sensors when the methane concentration in the mine varies due to different gas dynamic factors; nor do the conventional methods make it possible to find out the causes of malfunctions and pinpoint parts and units rendered inoperative.
The existing methods cannot be carried out with the aid of remote control systems and require that the checking personnel should descend underground to inspect the sensors.
There is known a method for checking the correctness of readings of a thermocatalytic sensor of a remote methane concentration measuring and gas protection system of a mine at reference points of its calibration curve. The method comprises purging of the reaction chamber with clean air and standard methane-air mixture (cf. "Rukovodstvo po oborudovaniyu i ekspluatatsii system avtomaticheskoy gazovoy zashchity i tsentralizovannogo telecontrolya soderzhaniya methana AMT-3 na ugolnykh shakhtakh" /"AMT-3, Automatic Gas Protection and Centralized Remote Methane Concentration Measuring System for Use in Coal Mines. Description and Service Manual"/, the Ministry of Coal Industry of the USSR, Moscow, 1974, p. 33). The purging with clean air serves to check the zero position of the sensor, while the purging with methane-air mixture is used to check the readings of the sensor at one point of its calibration curve.
However, the method under review is not intended for remote-controlled supply of information, but, on the contrary, makes it necessary to transport vessels containing clean air and standard methane-air mixture to sensor locations. Besides, the checking of sensors is inadequate in that it does not include the checking of operating speed which is a vital factor determining the dynamic error of methane concentration measurements with the methane concentration varying at different rates. Nor does the method under review make it possible to find out the causes of sensor failures whereof the most frequent ones include: variations in the throughput rate of the working thermoconverter element in the course of methane oxidation due to changes of the catalytic activity of that element under the effects of mine atmosphere components which poison the catalyst, or due to other reasons; and variations in the resistance of the reaction chamber to the transfer of methane through its gaseous exchange walls which may get soiled or be affected by dust and moisture.